1. Field of the Invention
The present invention relates to a valve timing control apparatus that controls valve timing of at least one valve of an internal combustion engine, which is driven by a camshaft through transmission of a torque from a crankshaft of the internal combustion engine.
2. Description of Related Art
One previously proposed valve timing control apparatus uses a phase adjusting mechanism, which is connected to a motor shaft of an electric motor, to adjust a relative phase (hereinafter, referred to as an engine phase) between a crankshaft and a camshaft according to a rotational state of the motor shaft. For example, Japanese Unexamined Patent Publication No. 2004-350446 teaches a valve timing control apparatus, in which switching elements are connected to stator coils of an electric motor that generate a magnetic field upon energization thereof. When the switching elements to be turned on are sequentially changed, the motor shaft, on which the generated magnetic field is applied, is rotated. In this way, the motor shaft is rotated according to the on/off control of the switching elements, so that the valve timing, which is determined by the engine phase, can be appropriately adjusted in response to the operational state of the internal combustion engine.
In the valve timing control apparatus recited in Japanese Unexamined Patent Publication No. 2004-350446, the motor shaft is rotated in the generated magnetic field of the stator coils, so that the induced voltage is generated in the stator coils. At this time, in a case where an actual rotational direction of the motor shaft coincides with a target rotational direction of the motor shaft, which is used to determine the switching order of the switching elements to be turned on, an induced voltage is generated in an opposite direction that is opposite from that of the applied voltage, which is applied to the stator coil through the turning on of the switching element. In this way, the electric current, which corresponds to a difference between the applied voltage and the induced voltage, flows through the corresponding on-state switching element, which is placed in the on-state.
In contrast, in a case where the actual rotational direction of the motor shaft does not coincide with the target rotational direction of the motor shaft, an induced voltage is generated in the same direction as that of the applied voltage, which is applied to the stator coil through the turning on of the corresponding switching element. In this way, the relatively large electric current, the amount of which corresponds to the sum of the applied voltage and the induced voltage, continuously flows through the on-state switching element that is placed in the on-state. Therefore, this switching element may be excessively heated to cause a thermal failure of the switching element.